The invention concerns a safety system for the drive of a cross cutter for cutting a web of goods, having an electrical or mechanical synchronizing system between the drives of the feed means and the cutting means which can be adjusted to the size of the piece to be cut, consisting of an adjustable asymmetrical mechanism (coupler mechanism) associated with the drive of the cutting means, and having a speed regulator associated with the drive of the feed means which limits the speed of the feed means on the basis of a signal corresponding to the setting of the asymmetrical drive.
Synchronized cross cutters have the purpose of dividing a web of goods, for example paper, into single sheets, by means of shear cuts made transversely of the direction of movement of the web. In order for the shear cuts to be made cleanly, it is necessary that synchronism exist between the web of material and the knives of the cutting means consisting of two rotating cutter rolls. This synchronism is brought about at a web velocity determined by the feed means consisting of two rotating rolls by associating an asymmetrical mechanism in the form of a coupler mechanism. By means of such a coupler mechanism, a uniform rotatory movement is transformed to a periodically varying rotatory movement. Accordingly, in the case of sheets which are to be cut to a length that is greater than the circumference of the cutter rolls, the cutter rolls will lag behind the web of material outside of the cutting phase, while in the case of sheets which are to be cut shorter than the circumference of the cutter rolls they will lead it. The asymmetrical mechanism thus permits synchronism in the cutting phase independently, to a great extent, of the size of sheets to be cut. Such a synchronous cross cutter, however, cannot operate at the same maximum web velocity at each setting of the asymmetrical mechanism, because the greater the degree of asymmetry that is preset in the asymmetrical mechanism, the greater will be the acceleration and the greater, therefore, will be the stress on the asymmetrical mechanism.
A variety of safety systems are known for preventing the capacity of an asymmetrical mechanism from being exceeded. In a known safety system of this kind (DT-OS No. 2,554,894), when the asymmetrical mechanism is preset, a voltage is adjusted on a potentiometer which is varied by means of a function generator according to the relationship between the size to be cut and the asymmetrical setting. This varied value is compared with the web velocity in an integrating circuit. If in this comparison permissible limits are exceeded, a trouble signal is delivered to the speed regulator of the feed means, so that the web velocity is reduced or no further increase is permitted in the velocity. To improve the reliability of this safety system, an additional function generator having an integrating circuit is connected in parallel with the function generator and the integrating circuit. These two parallel circuits are monitored for errors by the fact that the two outputs of the integrating circuits put out a trouble signal through an AND gate.
Although such a safety system has two parallel control circuits, it does not satisfy the safety requirements. It is disadvantageous that in order to limit the velocity of the web of goods the output signal of the integrating circuit is delivered as a trouble signal to the speed regulator of the feed means. Thus the danger exists of "hunting" in the regulation system leading to cutting length errors. It is also disadvantageous that two parallel circuits are required for the improvement of reliability. Another important disadvantage is that the function generator or generators can be adjusted only after the cross cutter starts up, because they are to deliver trouble signals matched to the speed regulator of the feed means.